Conventional cancer treatments lack target specificity and are therefore associated with severe side effects. Highly specific, targeted cancer treatment approaches are urgently needed. We propose a novel theranostic approach that uses mesenchymal stem cells (MSCs) expressing a cell surface antigen to specifically target a cytotoxic radioisotope to cancer cells. The cell surface antigen also allows monitoring targeting of MSCs with Positron Emission Tomography. We will test the hypothesis that MSCs expressing a cell surface antigen (DAbR1) can home to tumors using PET imaging with 86 Yttrium- (S)-2-(4-acrylamidobenzyl)-DOTA (86Y-AABD). Subsequently, we will label AABD with 90Y (a high energy beta-emitter) to induce local and selective radiation. Thus, we propose a novel theranostic approach in vivo. This work leverages tools and reagents developed by UCLA investigators; we have developed a cell membrane expressed antibody fragment called DAbR1 that can be detected with positron emission tomography (PET) using 86Y-AABD. AABD can also be labeled with 90Y for radiation treatment. The theranostic will be used as follows: i) MSCs expressing DAbR1 will be injected into tumor bearing mice. ii) Homing of optimized quantities of DAbR1-MSCs to tumors will be confirmed by 86Y-AABD PET imaging. iii) 90Y-AABD will be injected only after PET imaging has confirmed optimal homing of DAbR1-MSCs to tumors to deliver selective tumor irradiation. The team of proposed investigators has combined excellence in cancer biology, molecular biology, biochemistry, radiochemistry, and preclinical and clinical molecular imaging. In Aim 1 we will analyze the trafficking of DAbR1 expressing MSCs to tumor xenografts in mice with 86Y-AABD PET. We will determine whether genetically engineered stably expressing DAbR1 retain their normal proliferation and differentiation. We will then assess the whole-body biodistribution of DAbR1 expressing MSCs at multiple time points with 86Y-AABD small animal PET/CT in mice bearing abdominal and subcutaneous tumor xenografts. In Aim 2 we will test the therapeutic effects of 90Y-AABD following tumor infiltration of DAbR1 expressing MSCs. The ultimate objective of this proposal is the development of a novel theranostic system, to be used for highly specific cancer radiation therapy with the radiopharmaceutical 90Y-AABD.